Methods and systems for cleaning heat exchange surfaces of a heat exchange system

ABSTRACT

Means for obtaining accurate knowledge of location and amount of fouling inside a heat exchange system, such as a boiler of a power plant, are provided. According to the invention this knowledge can be used to optimize cleaning of a heat exchange system. The system of the invention comprises: Means for measuring particles in the exhaust gas stream of the heat exchange system. These particles are at least partly released when cleaning a certain part of the heat exchange surface of the heat exchange system. Means for creating information of the fouling in an electronic memory by linking together coordinates of the part of the heat exchange surface being cleaned and the measurement data created during the cleaning of said part.

CROSS-REFERENCE

This application is a continuation of commonly owned copending U.S.Application

Ser. No. 10/549,280 filed on Sep. 13, 2005 (U.S. Pat. No. ______), whichis the national phase application under 35 USC §371 ofPCT/FI2004/000190, filed Mar. 31, 2004 and claims the benefit ofpriority from U.S. Provisional Application Ser. No. 60/458,442, filedMar. 31, 2003, the entire content of each being hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates generally to process industry, such aspower plants. Particularly the present invention relates to determiningfouling in a heat exchange system and method of cleaning such a heatexchange system, such as a boiler of a power plant. More particularly,the present invention relates to a method for air/fuel control.Furthermore the present invention relates to a method for optimizing ofcleaning particles or fouling from surfaces of a process system.

BACKGROUND OF THE INVENTION

It has been known for a long time that maintaining the stoichiometricratio between air and fuel in a pulverized fuel fired process is animportant criterion to minimize emissions such as NO_(x) and CO. Forexample, a pulverized coal (PC) boiler constitutes a large number ofburners. It has been both observed and proved that the stoichiometricratio between air and fuel has to be maintained on a per burner basis.Therefore, both the fuel flow and the airflow are measured, and eitherthe airflow or the fuel flow is used as the control variable, to keepthe ratio between the fuel flow and the airflow for each individualburner within strict limits.

It has been proved that matching the airflow to the fuel flow for eachindividual burner reduces emissions as well as improves other variablesin the operation of a solid fuel fired boiler. However, it has beenidentified that only matching the airflow and the fuel flow does notprovide the minimum emission level for the NO_(x). CO and other adjacentemissions.

The typical and current air/fuel balancing concept has been described inthe Scheme of FIG. 1. As illustrated in FIG. 1, typical air/fuelbalancing method is based on air flow and coal flow measurements foreach individual burner. Please note, that the total amount of air ismatched with the total amount of fuel by keeping the O₂ concentration inthe exhaust gas on a certain level (e.g. 2%). The point of the knownoptimization methods is to keep the same share of fuel and air on eachburner. If one burner carries a higher amount of fuel, a higher amountof air should be distributed to that burner. That is, the percent fueland the percent air on one burner should be the same. However, now ithas been surprisingly observed that occasionally either more or less airthan the stoichiometric ratio would suggest, is needed for a certainburner in order to minimize the emissions. The reason for thisphenomenon is unknown, but has most likely a connection to the mixingproperties of the fuel and air in the flame. Therefore, a need exists inthe industry for a method of optimizing air/fuel ratio whereinoptimization will be made more efficiently being based on themeasurements of the actual process conditions.

The present invention relates further to soot cleaning optimization.Minimizing of emissions such as NOx, decreases also the need forsoothing. Cleaning particles (fouling) from surfaces is a routine thatis fairly common in the process industry. For example, when running acombustion process it is essential to keep heat exchanger surfaces cleanfor the sake of efficiency. Many different kinds of soot cleaners(blowers) are used and they are run according to a certain sequence tokeep the heat exchange surfaces as clean as possible. The soot cleaningis generally done by blowing steam on the heat transfer surfaces or byusing pressurized air or sound waves to remove the particle layer,mainly soot from the heat transfer surfaces. The particles released fromthe heat transfer surface section that is soot blown are then entrainedinto the exhaust gas stream.

Running soot cleaners is expensive. Furthermore, cleaning heat exchangertubes with steam, without any particle layer on their surfaces, is veryeroding for the walls of these tubes. Erosion of the heat exchangertubes is again a very expensive affair. However, high expenses willemerge as well if soot cleaners are not used at all. Therefore, it is ofgreat importance to optimize the soot cleaning process thoroughly.

Typically the need for the soot cleaning is estimated from raisedexhaust gas temperatures and possible steam temperature anomalies. Somesystems weight the heat transfer tubes and on the basis of the mass ofthe tubes estimate the amount of the fouling on the tubes. Informationobtained by these methods does not necessarily give the preciseinformation about which heat exchanger tubes has the most part of thesoot stuck to its surface and which tubes are fairly clean.

Therefore, a need exists in the industry for a method of optimizing sootcleaning whereby the soot cleaning will be made more economically andefficiently being based on the measurements of the actual processconditions.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention is to provide a method for air/fuelcontrol wherein at least one of the group of primary airflow, millparameters, and secondary airflow is controlled using a controlalgorithm, which is determined by correlation analysis between ECTsignals and the output and input signals of the process in order todetect dependencies, and by fuzzy modeling of the dependencies.

Furthermore, an another object of the invention is to provide a sootcleaning optimization method to be used in a process industry in whichinformation on a sequence of a cleaning, time between running, etc.variables for cleaning devices are optimized based on the measurement ofthe particles entrained in the gas stream of the process. Themeasurement is based on detecting static electricity and/or changethereof in the gas stream of the process.

Another object of the invention is to provide means for obtainingaccurate knowledge of location and amount of fouling inside a heatexchange system, such as a boiler of a power plant. According to theinvention this knowledge can be used to optimize cleaning of a heatexchange system.

A typical method in a heat exchange system according to the inventioncomprises following steps:

-   -   Exhaust gas steam is led by a heat exchange surface of the heat        exchange system.    -   A certain part of the heat exchange surface of the heat exchange        system is cleaned with a cleaning equipment having an operation        parameter status. A typical cleaning equipment of the invention,        e.g. a steam based soot blower in a boiler, is arranged to clean        a certain part of the heat exchange tubes in the boiler. A        typical large boiler comprises several separate pieces of        cleaning equipment, each of which can typically be run        separately of each other. A typical steam based soot blower in a        boiler blows steam of a certain pressure on the heat exchange        tubes to be cleaned and is moved over its part of the tubes at a        certain point of time, with a certain speed. These operation        parameters can normally adjusted by the operator of the boiler.    -   Particles are released from the heat exchange surface. Normally        and mostly these particles are soot. Soot is formed on different        parts of the heat exchange surfaces with different speeds        depending on various process parameters, e.g. the type and        amount of fuel used. The amount of particles released from a        certain part of the heat exchange surfaces by the cleaning        equipment depends e.g. on the steam pressure of the cleaning        equipment and the amount of particles that has been formed on        that certain part being cleaned. The time elapsed between two        cleanings of the same heat exchange tubes naturally effects on        the amount of impurities formed on the tubes.    -   The released particles are led into the exhaust gas stream of        the heat exchange system.    -   Amount and/or type of the released particles in the exhaust gas        stream is measured and particle measurement data of these        particles is created on the basis of these measurements. These        measurements can be done with different kinds of equipment.        Examples of a measurement systems and methods suitable for this        purpose are given in the applicant's earlier patent publications        U.S. Pat. No. 6,031,378 and WO 02/06775. That system is called        Electric Charge Transfer System, or ECT-system. Suitable parts        of these publications are hereby incorporated in this text by        reference. In one embodiment of the invention the mass flow of        particles in the exhaust gas stream is measured.    -   Information of the fouling is created in an electronic memory by        linking together and storing in the electronic memory        coordinates of the part of the heat exchange surface of the heat        exchange system being cleaned and the measurement data created        during the cleaning of said part.

A typical system for determining fouling in a heat exchange systemaccording to the invention comprises means that enable the method of theinvention, i.e.:

-   -   Means for detecting operation parameter status of a cleaning        equipment arranged to clean a certain part of the heat exchange        surface of the heat exchange system. Naturally, these means        should provide the system with the status of the wanted        operation parameters in electronic form.    -   Means for measuring the amount and/or type of released particles        in the exhaust gas stream of the heat exchange system, e.g. the        above-mentioned Electric Charge Transfer System, or ECT-system.    -   Means for creating particle measurement data of released        particles in the exhaust gas stream. This is normally a runnable        computer program on e.g. the memory of a PC or any other        suitable computer.    -   An electronic memory e.g. on the PC,    -   Means for creating information of the fouling in the electronic        memory by linking together and storing in the electronic memory        coordinates of the part of the heat exchange surface of the heat        exchange system being cleaned and the measurement data created        during the cleaning of said part. This means that a database is        created e.g. on the hard disc of the PC. This database can then        be used in many different ways to examine the fouling.

The system of the invention can comprise e.g.:

-   -   Electronic means for analyzing the information of the fouling        and for creating control signal for the cleaning equipment of        the heat exchange system. This means e.g. a computer program        used to analyze the information of the fouling in the electronic        memory and signaling means from said computer to the cleaning        equipment.

The operation parameter status of the cleaning equipment that isdetected and stored in the electronic memory typically comprises statusof at least one and preferably several of the following operationparameters:

-   -   Identification data of the cleaning equipment. The piece of        cleaning equipment used at any time should be clearly        identifiable.    -   Coordinates of the cleaning equipment in the heat exchange        system.    -   Operational status of the cleaning equipment, i.e. is the        cleaning equipment running or not running,    -   Speed of the cleaning equipment.    -   Information on the effect of the cleaning equipment, e.g. steam        pressure used.

The most important piece of information to know is from which part ofthe heat exchange surfaces the particles measured in the exhaust gasstream were released. Knowledge about fouling tendency, i.e. the amountof fouling formed on different parts of the heat exchange system isobtained with this information. Typical suitable soot blower equipmentcomprises at least one of the following types of devices:

-   -   steam based soot blower    -   acoustic soot blower    -   air gun.

Other possible cleaning equipment suitable for use in the method andsystem of the invention are:

-   -   hammer cleaner    -   mechanical cleaner, such as steel-wire brush.

These different kinds of cleaning equipment are suitable for differentcircumstances.

In an embodiment of the method according to the invention theinformation of the fouling stored in the electronic memory is processedas a function of the heat exchange surface coordinates. Typically hisprocess comprises optimization steps in order to find at least one ofthe following optimal parameters:

-   -   an optimal time to start cleaning of a particular part of the        heat exchange surface of the heat exchange system    -   optimal cleaning speed for a cleaning equipment of a particular        part of the heat exchange surface of the heat exchange system    -   optimal operation parameters for the cleaning equipment for        cleaning a particular part of the heat exchange surface of the        heat exchange system.

In an embodiment of the invention the aforementioned optimization isbased on one or more of the variables:

-   -   time to be elapsed between two cleanings of a particular part of        the heat exchange surface of the heat exchange system    -   fouling tendency of ash on a particular part of the heat        exchange surface    -   carbon content in ash.

As a result of this kind of optimizations more efficient cleaning of theheat exchange system is achieved.

In further embodiments of the invention the information of the foulingstored in the electronic memory is used for at least one of thefollowing:

-   -   Estimating fouling tendency on the heat exchange surfaces as a        function of heat exchange surface coordinates. This means        information about how easily fouling is formed on a certain        location on the heat exchange surfaces.

Estimating fouling distribution on the heat exchange surfaces as afunction of heat exchange surface coordinates. This means informationabout how much fouling is there on a certain location on the heatexchange surfaces.

As a result of this kind of estimations cleaning of the heat exchangesystem can be planned to be more efficient.

In an embodiment of the invention

-   -   particle distribution on a cross-section of the exhaust gas        channel is measured    -   the measured data of the particle distribution is compared on        previous measurements of the particle distribution    -   fouling tendency and location for the fouling in the heat        exchange system is determined by utilizing the comparison.

The particle distribution on a cross-section of the exhaust gas channelgives knowledge, when compared with previous results, about the originof the particles. The afore-mentioned Electric Charge Transfermeasurement system is very suitable for these particle distributionmeasurements. With help of the ECT system fouling tendency and locationfor the fouling in the heat exchange system are determined in anaccurate manner. Also the amount of unburned carbon in the ash flow inthe exhaust gas stream can be estimated using signals produced by theECT measurement system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of an example and is notlimited in the accompanying figures, in which alike references indicatesimilar elements, and in which;

FIG. 1 illustrates schematically an air/fuel balancing concept accordingto the prior art,

FIG. 2 illustrates schematically a flow scheme of correlation analysisaccording to the present invention,

FIG. 3 illustrates schematically a fuzzy modeling algorithm according tothe present invention,

FIG. 4 illustrates schematically an implementation of a control systemaccording to the present invention,

FIG. 5 illustrates a schematic embodiment of an arrangement according tothe present invention,

FIG. 6 illustrates a block scheme of an optimization according to thepresent invention, and

FIG. 7 illustrates a simplified block scheme of a soot cleaning methodaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the first aspect of the invention provides a method forair/fuel control in burners, such as pulverized coal boiler, based on ameasurement of a flow of particles for a suspension of gas and solids.The measurement can be used e.g. by using the measurement systemdisclosed in the applicant's earlier patent publication U.S. Pat. No.6,031,378 and/or the method disclosed in the applicant's earlier patentpublication WO 02/06775. The measurement system (Electric ChargeTransfer System, ECT-system), disclosed in the above-mentioned patentpublications, is able to measure e.g. the velocity and the mass flow ofparticles for a suspension of gas and solids. The ECT measurement is ofa local character, that is, the signal caused by the flowing particlesis a function of distance from the particles to the ECT antenna.Therefore, a big duct normally requires use of many ECT antennas. Itshould be noticed that the particles entrained in the gas flow are notnecessarily evenly distributed over the whole duct. Using severalantennas will ensure that the particle flow is sensed properly over thewhole duct, even though the rope of the particles would change itscoordinates. Please note that a not even distribution of ash particlesin the exhaust duct contains also a lot of valuable information.

The ECT system measures the state of the two-phase flow in burner ducts.The ECT measurement splits the raw signal (ECT LF signal) into AC and DCcomponents. DC component is the spectral line for ˜0 Hz (mean value).Normal AC is the standard deviation of the raw signal on the frequencyband 0.3-15 Hz. The ECT velocity measurement collects measurementsignals with a high sampling frequency (22 kHz). Fans and compressors aswell as the combustion process (flame) cause pressure gradients in thegas flow. These gradients can be seen as intensified spectral density ondifferent frequencies on the raw signal (ECT HF signal).

It has been observed that some patterns in the above mentioned ECTsignals correlate with the readings from the emission metering devicesof the boiler. The ECT measurement will provide information whether moreor less air is needed than the stoichiometric ratio between fuel and airwould suggest. The methodology to determine the optimal dosing of airfor a burner is explained in more detail below.

The ECT signals (HF and LF) mirror the flow properties in the burnerducts. These flow properties depend on the process variables such asparticle size, mass flow, particle velocity, and the flame properties(flame properties affect mainly the ECT HF signals). Dependency betweenthe ECT signals and the output signals (NOx, CO, O2, airflowmeasurements, etc.) is estimated with different methods. The followingmethods can be used: correlation analysis, spectral analysis and fuzzymodeling. The result will be a dependency matrix showing which burner(s)has the strongest connection to the emission rates (e.g. NOx and CO).

The correlation analysis will typically build up large correlationmatrixes between the ECT variables for the different burners as well asbetween the ECT variables of burners and the output variables (NOx, CO,O2, etc.). The size of the matrixes can be reduced significantly byeliminating such ECT signals that have strong correlation to a chosenECT signal. In order to reduce the size of the matrix, a loop between 1and n (number of burner pipes) is established where j expresses thereference burner pipe and k is the burner pipe against which thecorrelation is checked. If the correlation is strong enough between theECT signals of the pipe j and the pipe k, the pipe k can be eliminatedfrom the matrix due to the fact that the ECT signals for the pipe k isrepresented in the ECT signals in the pipe j because of the strongcorrelation. This method will reduce the size of the matrixes and wouldalso make it possible to group different burner pipes according to theirinternal correlation. Please see the flow scheme illustrated in FIG. 2(R shows the correlation).

Spectral analysis is applicable only on signals that have a well-definedsampling rate. This is not the case for many of the output measurementsused in prior art methods, which are based on the principle of taking asample and analyzing it offline. Also time of update for thesemeasurements can be even a few minutes.

The most potential signal for spectral analysis purposes is the ECT HFsignal for each pipe because this signal type reflects well the state ofthe flame. Please note that two individual channels are used for eachpipe to get the particle velocity. The flame impacts the ECT HF signalsstrongly besides the fans that transport the gas into the boiler as wellas out from the boiler.

The spectral analysis will divide the ECT HF signals into differentbands and determine which of the bands are correlating with the flamequality, and which of the bands are also related to other variables suchas particle size, mass flow of the coal etc. The standard deviation willbe calculated for each band and stored as a variable in a matrix.

When the ECT-system is used, there are a lot of signals available withdifferent properties. The key issue is to be able to determine thedependency between these signals in a reliable and simple way. Fuzzylogic rules fulfill these criteria. The noise has to be removed from thesignal without loosing any relevant information in the signal. Thealgorithm works roughly as illustrated in FIG. 3 for each measurementvector.

The air/fuel control method according to the present invention canfavorably be added on top of the air to fuel balancing in order to gainmore reduction in the emissions. The control variables that can be usedare fairly limited. The main control variables to affect the process areas follows: primary airflow (PA), mill parameters (separator settings,etc.) and secondary airflow (SA).

Please note that the role of the primary airflow is to transport thecoal to the furnace, and the primary air should usually be kept as lowas possible.

Therefore, this variable does not usually offer much controllability,but the primary air should be high enough to provide a proper transportof the coal.

Mill parameters such as separator settings etc. are important in orderto keep the particle size of the coal as small as possible and the flowas steady as possible. However, there are only static classifiers(separators) on many plants, which limits the use of the separatorsettings as a control variable. It should be noticed that the steadyflow of the fuel and a small particle size are essential for an optimalcombustion.

The most favorable variable to be used for minimizing the emissions isusually the secondary air (SA). The SA has a great impact on the flame,and hence, also impacts the ECT HF signals strongly. The block scheme asillustrated in FIG. 4 shows the control structure roughly.

Generally, the second aspect of the present invention provides anoptimized soot cleaning process based on a measurement of a mass flow ofparticles for a suspension of gas and solids. One process of this kindis illustrated as a simplified block scheme in FIG. 7. The measurementcan be used e.g. by using the measurement system disclosed in theapplicant's earlier patent publication U.S. Pat. No. 6,031,378 and/orthe method disclosed in the applicant's earlier patent publication WO02/06775. Other suitable measuring systems are for e.g. other electricalmeasuring systems and optical analyzing systems. The soot cleaningoptimization method can be utilized also independently in processes inwhich method for air/fuel control according to the first aspect of theinvention is not used.

When the soot cleaning (particle cleaning) is in operation there will bemore particles entrained in the gas stream than normally. The increasein the concentration of the particles will be calculated based on theincrease in the ECT reading during the soot cleaning. Please see theillustration in FIG. 5 describing the arrangement. It should be noted,that the soot cleaning method according to the present invention can becarried out by using also other suitable measuring systems than ECT andwhich can detect changes in the gas stream during the soot cleaning.Such systems include e.g. optical measuring systems and other electricalmeasuring systems, such as systems using laser or acoustic waves.

The dependency between each cleaner and the ECT reading is mapped. Thismeans in practice that the amount of particles that has built up in thecoverage of a cleaning device k is calculated from the ECT readings.

m _(k) =f(ECT)/T _(k)  1

where:

-   -   m_(k)=particle mass flow when cleaner k is running    -   T_(k)=time elapsed between the last run of cleaning unit k

It should be noticed that the signals from advantageously all ECTantennas will be used for calculating the mass of particles that areemerged into the gas stream by cleaning unit k. In a situation whereseveral cleaners are running simultaneously, a multivariable correlationanalysis is to be applied.

The main variable that is to be optimized is the time (T_(k)) betweenthe run of each cleaning device k (k=1,n, where n is the number ofcleaning devices). This procedure is fairly straightforward. A limit(M_(LK)) for how big the m_(k) is to be for cleaning is defined. TheT_(k) is then extrapolated from the latest run of the cleaning unit k,by also noting other process variables such as gas flows, solid feeds,etc.

Besides the elapsed time between the run of the cleaning unit, also theruntime and other parameters concerning the cleaning device is to bedetermined in order to achieve a maximal cleaning efficiency. The objectfunction for each cleaning device depends on the physical properties ofthe device and should, hence, be determined on a case by case basis.

Furthermore, it has been observed that a certain signal behaviorreflects specific conditions for the particles passing the antennamatrix. For example, a positive DC signal on a normal AC level indicatesa higher content of carbon in the ash flowing past the ECT antennamatrix. If the particles show a high negative DC signal on a normal AClevel, the particles possesses properties that enable them to easily tostick onto the surfaces. Hence, ECT signal can be used to estimateimportant properties for the ash flowing in the exhaust gas channel.Please note that a high carbon in ash indicates a poor combustion andhence a risk for fouling.

The concept according to the present invention is used to optimize thesoot cleaning more thoroughly. The block scheme in FIG. 6 illustratesthe procedure. At least partly based on ECT measurements, one canestimate one or more of the following variables: 1) a time to be elapsedbetween runs of cleaning units k, 2) fouling tendency of the ash, and 3)carbon content in ash. Beside said estimates, one can use one or more ofthe following attributes as a variable in optimization: a) data input(temperatures, steam date, etc.) from data collection system of theprocess, b) data base containing history from previous cleaning andresults, and c) ECT measurements. According to the present invention, bycombining desired values from the group of estimated variables 1-3 andvariables a-c, optimization of the soot cleaning process can be made. Anaim of the optimization process is to maximize the efficiency of theprocess, such as the combustion process, and to minimize the costs ofthe cleaning process. As a result from the optimization process, oneachieves information which can be used to control the cleaning sequence,time between running of cleaning devices, or the like variables for thecleaning devices.

The present invention provides an improved control for the soot cleaningprocess. Based on the information achieved with the optimizationaccording to the present invention, one can e.g. define individually foreach separate cleaning device different time between running and runningparameters during cleaning.

While the invention has been described in the context of a preferredembodiment, it will be apparent to those skilled in the art that thepresent invention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.The air/fuel optimization method and the soot cleaning optimizationmethod can be exploited independently and thus described methods are notdependent of each other. Furthermore, it should be noted, that the sootcleaning method according to the present invention can be carried out byusing also other suitable measuring systems than ECT and which candetect changes in the gas stream during the soot cleaning. Such systemsinclude e.g. optical measuring systems and other electrical measuringsystems.

1. Method of air/fuel control of burners in a pulverized coal firingboiler which comprises controlling input parameters of the boiler on thebasis of a measurement of a flow of particles in burner ducts.
 2. Themethod according to claim 1, wherein the input parameters controlled onthe basis of the measurement comprise at least one of the group of millparameters, primary airflow of the burners and secondary airflow of theburners.
 3. The method according to claim 2, wherein the inputparameters controlled on the basis of the measurement comprise secondaryairflow of the burners.
 4. The method according to claim 1, wherein themeasurement comprises measuring ECT signals.
 5. The method according toclaim 4, wherein the controlling is based on spectral analysis of theECT signals.
 6. The method according to claim 5, wherein the controllingis based on a high frequency (HF) component of the ECT signals,reflecting the state of the flame in the boiler.
 7. The method accordingto claim 4, wherein the controlling is based on ECT signals obtained byseveral antennas so as to measure a particle distribution in the burnerducts.
 8. The method according to claim 4, wherein the controlling isbased on a correlation analysis between the input parameters, the ECTsignals, and output parameters of the boiler including readings of atleast one emission metering device of the boiler.
 9. The methodaccording to claim 5, wherein the controlling is based on a correlationanalysis between the input parameters, high frequency (HF) bands of theECT signals, and output parameters of the boiler including readings ofat least one emission metering device of the boiler.
 10. The methodaccording to claim 8, wherein the size of a correlation matrix obtainedin the correlation analysis is reduced by eliminating ECT signals havinga high correlation with another ECT signal.
 11. The method according toclaim 8, wherein the controlling is based on fuzzy modelling of thedependencies between the ECT signals and the output parameters.
 12. Themethod according to claim 9, wherein the controlling is based on fuzzymodelling of the dependencies between the high frequency (HF) bands ofECT signals and the output parameters.